The moisture content of a sample is determined by drying the sample and by manually determining its weight before and after the drying process. Due to the laborious procedures involved, this method is very expensive and error-prone.
Under certain conditions, the weight loss can also be determined during the drying process. For a given sample, the measured value for the weight decreases as a function of the temperature, the elapsed drying time, and the parameters of the testing compartment, according to a weight vs. time profile graph which asymptotically approaches the dry weight of the sample. The graph for the sample under examination can be determined through comparison experiments and expressed mathematically in the form of an approximation formula. Based on the available electronic capabilities, an appropriately equipped measurement instrument for the gravimetric determination of moisture content is able to calculate the moisture content of a sample from the measured parameters of the profile graph and the drying time and to indicate the moisture content on a display unit. With this method, it is no longer necessary for the material under test to be completely dried out, because sufficient information is obtained by determining the coordinates of two measurement points in the weight/time diagram.
As mentioned above, the change in the weight of a sample is essentially a function of the temperature, the elapsed drying time, and the parameters of the testing compartment. The instruments that are commercially available at this time are limited in their accuracy primarily because of the demanding requirements which have to be met by the testing compartment.
Normally, the sample is spread in a thin layer onto a flat sample receiver, for example a sample tray. The tray is arranged inside the gravimetric moisture determination instrument with the sample-receiving surface extending horizontally and parallel to the heat-emitting surface of the sample-heating means, in order to achieve a uniform heating of the sample.
Devices that are used as sample-heating means include a variety of radiation sources such as radiant heaters, microwave generators, halogen- and quartz lamps. As could be established by experiments, one of the main causes for the inaccuracy in the determination of measurement values in existing gravimetric moisture-determination instruments lies in the nature of the radiation sources being used and in their arrangement in the testing compartment.
A measuring instrument of the aforementioned kind for the gravimetric determination of moisture content is described in the European Patent EP 0 611 956 B1. In this instrument, the loading of the weighing pan occurs outside of the gravimetric moisture-determination instrument. To load and unload the sample, the balance which is arranged on a drawer-like carriage is slid out of the instrument housing. As a radiation source, a ring-shaped halogen lamp is being used which, in the operating state of the apparatus, is arranged above the sample receiver. Examples of further design configurations are published in EP 1850 110 A1. For example, a measuring instrument is disclosed which is opened up for loading through a horizontal swivel movement of a part of the housing. In a further embodiment, a part of the housing is swiveled in the vertical direction. Likewise, the measuring instruments disclosed in U.S. Pat. No. 7,851,712 B2 are opened for loading by vertically swiveling a part of the housing.
All of these types of designs have the disadvantage that their radiation sources, hereinafter also referred to as sample-heating means, can be hot and therefore pose a risk of injury to the user. Furthermore, a technician servicing the equipment can be exposed to the danger of an electric shock.
It is therefore the object of the present invention to provide a measuring instrument for gravimetric moisture determination which offers the assurance of user safety.